1. Field of the Invention
This invention relates to a drag bit having stud-mounted polycrystalline diamond (PDC) cutting elements and more particularly, to an improved structure for securing the studs elements within the body of the bit.
2. Description of the Prior Art
Stud-mounted polycrystalline diamond cutting elements are readily commercially available, one such being a STRATAPAX cutting element from General Electric. Such PDC cutting elements generally comprise a cylindrical stud of tungsten carbide having an angular planar face formed at one end of the stud. A generally cylindrical wafer of tungsten carbide is brazed onto the planar face with the opposite exposed face of the wafer comprising a layer of polycrystalline diamonds.
In fabricating a drag bit utilizing such PDC cutters, the body of the bit has cavities open to the surface with such cavities generally corresponding to the diameter of the cylindrical stud. The PDC cutter is then oriented within the cavities such that the polycrystalline diamond surface faces in the direction of rotation of the bit. Although in some bit bodies the studs are press-fit within the cavities, it is also well known to retain the studs by brazing, with a well known brazing material, such as by wrapping the studs with a foil of the brazing material prior to insertion in the cavities and properly orienting them prior to subjecting the bit body and assembled cutting elements to a brazing temperature.
The angular planar surface formed in the stud on which the wafer is mounted, and the exposed exterior end of the tungsten carbide stud, which is rounded to generally conform to the cylindrical surface of the wafer mounted on the planar surface, provide surfaces which, when the stud is seated within the cavity, terminate below the surface of the bit body such that the dissimilar geometries of the bit body and such surfaces define various voids below the surface of the bit and adjacent the internal wall of the cavity. During boring or drilling conditions, wherein highly abrasive drilling fluid is circulated across the bit face and adjacent the cutters to remove the cuttings and cool the cutter, the high velocity fluid enters these voids with such velocity that it causes erosion of the bit body and also permits erosive attack of the brazed joint between the bit body and cutting element in those bit bodies in which the stud is secured by brazing. Such action ultimately washes out the support around the cutter to the extent that quite often the cutter is released from the bit body with substantial deleterious effect on the ability of the bit to continue drilling.